Geotagging is the process of adding Global Positioning System (GPS) location data to photographs or other media, enabling users to easily and accurately know where in the world the media was captured. For example, a geotag can be added to a photograph to show the location of the photograph at the time it was taken. Geotags can be added to any media format such as photographs, video, websites, SMS messages, and RSS feeds. A geotag associates a particular location with the media at the time it was generated. Among other things, geotagging can help users find a wide variety of location-specific information. Geotagging-enabled devices and services can also be used to find location-based news, websites, or other resources.
Geotags are metadata that can be associated with media data. A full geotag is a geotag that contains geographic data—such as latitude and longitude—capable of specifying GPS device position (and optionally velocity, altitude, orientation, accuracy, and heading), as well as the position of any device attached to, or in the vicinity of, the GPS device. The metadata format built into most modern digital cameras and wireless telephones supports the inclusion of this location data. For example, image files such as JPEG have standard ways of embedding location information into the metadata, via either EXIF (Exchangeable Image File Format) or XMP (Extensible Metadata Platform) standard formats. With photographs stored in JPEG file format, for example, the geotag information is embedded in the metadata. The common fields within the EXIF format for geotag data are GPSLatitude, GPSLongitude, and GPSAltitude. The metadata can also store custom data. An example readout for a JPEG photograph might look like:
GPS Latitude: 57 deg 38′ 56.83″ N
GPS Longitude: 10 deg 24′ 26.79″ E
Most image management and viewing tools are now capable of recognizing geotags.
The GPS location data of geotags is determined using a GPS device. Modern handheld devices such as digital cameras, mobile phones, and PDA devices may be equipped with built-in GPS receivers or may be configured for use with a GPS card or other plug-in GPS device. GPS receivers determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide a measure of range from the GPS receiver to each of the satellites that are in view of the receiver. GPS satellites transmit to the receivers both satellite-positioning data called “ephemeris” data and unique pseudo-random noise (PN) codes that identify the particular satellite and allow signals transmitted simultaneously from several satellites to be received simultaneously by a GPS receiver with very little interference of any one signal by another. The PN code sequence length is 1023 chips, corresponding to a 1 millisecond time period. One cycle of 1023 chips is called a PN frame. Each received GPS signal is constructed from the 1.023 MHz repetitive PN pattern of 1023 chips. At very low signal levels, the PN code pattern may still be observed to provide unambiguous time delay measurements, by processing, and essentially averaging, many PN frames. These measured time delays multiplied by the speed of light are called “pseudoranges.” These measured pseudoranges are included in information that becomes part of a partial geotag. It should be noted that the term pseudorange is the range plus the GPS receiver clock bias (with respect to GPS time) multiplied by the speed of light; however, for the sake of brevity and convenience, the terms range and pseudorange can be used interchangeably. In addition, satellite ephemeris data can also include almanac data, which is a reduced accuracy version of ephemeris data that provides reduced position accuracy. A set of four measured pseudoranges, when combined with knowledge of the absolute times of transmission of the GPS signals and respective satellite state at those times is sufficient to solve for the position of the GPS receiver. But the process of searching for and acquiring GPS signals and reading the ephemeris data for a plurality of satellites is time consuming and may introduce unacceptable delays in computing the receiver position. In addition, in many situations, there may be blockage of the satellite signals. In these cases, the received signal level can be too low to demodulate and derive the satellite data without error.
Media devices, such as digital cameras, often spend much of their time idle, and in locations where GPS reception is sporadic and signals are often too weak to decode the transmitted data stream from one or more satellites. Under these conditions, even with the application of various low power operating modes, the GPS receiver will often be unable to compute a position fix without a noticeable delay. Yet users often want a media device to be ready to capture a particular form of media instantly, at or very near the time when the media device is powered on. In many cases, the media will be captured indoors or in other severe environments where the ability to acquire strong GPS signals is limited. In such a case, the satellite state data, or ephemeris data, may not be available to compute a full geotag at the time the media is captured, or within a reasonably short amount of time later. In addition, the user may subsequently turn off the media device before the GPS receiver has been able to acquire the ephemeris data. The user may want a geotag immediately, but the GPS device is unable to provide one. Current GPS receivers have no way to resolve this problem, so the media will not be geotagged. The inability of GPS receivers to provide fast location fixes in severe environments without the need for assistance from a network, and to do so without excessively draining the device battery, have been obstacles to integrating geotagging with media devices.